Attack code exploiting a recently-patched vulnerability in Windows has been posted to the Internet, prompting concerns of a widespread attack.

The software was added to the widely used Metasploit project, a favourite of both security researchers and malicious hackers, yesterday, according to HD Moore, the Metasploit project leader.

"It works very reliably against Windows 2000 and Windows XP systems that do not have SP2 nstalled," he said.

Security experts were worried that the Windows Server services vulnerability - described in the latest Microsoft security bulletin - could be used in a widespread worm attack.

Windows Server services are generally enabled by default on Windows systems, and are used for common network applications like file sharing and printing.

The bug was patched on Tuesday in one of 12 Microsoft security updates.

On Wednesday the US Department of Homeland Security (DHS) took the unusual step of warning PC users to make sure they had installed this patch.

The DHS statement warned that the vulnerability "could impact government systems, private industry and critical infrastructure, as well as individual and home users."

"This is a great opportunity for an unskilled hacker to launch a worm," said Marcus Sachs, deputy director with research group SRI International's Computer Science Laboratory.

"A skilled hacker will use the vulnerability to quietly infect millions of computers for the purposes of sending spam, stealing credit card numbers, or countless other subversive activities," he said in an e-mail interview.

Microsoft executives were not immediately available to comment on the Metasploit code.

In a blog post dated early Thursday, Microsoft Security Response Center Program Manager Christopher Budd, said his company was seeing "very, very limited exploitation of the vulnerability."

Metasploit's Moore believes that any worm based on this vulnerability will probably not be as widespread as the Zotob worms, which made headlines last year after taking down computers at Cable News Network, SBC Communications and American Express.

The vulnerability exploited by Zotob "was actually much more reliable and affected a wider range of systems," Moore said. With this latest malware, "the only shops that really need to worry are those running older XP clients or 2000/NT desktops," he said.

Moore's public comments on the attack code can be found here.